"Every runner over 45 that I see in here has advanced osteoarthritis in his knees," my doctor told me last year before recommending surgery for my torn meniscus. "I tell all you guys the same thing: ‘The impact is too much. Switch to the elliptical or cycling.'" Instead, some runners take up impact-reducing techniques such as the Pose Method, ChiRunning, aqua-jogging or barefooting. Others eye innovative running machines — indoor and outdoor — that once might have been reserved just for rehabilitation and high-performance training. If you're addicted to the runner's high and want to save what's left of your cartilage before it's too late, one of these expensive contraptions might be worth the investment in the long run.

Running on a sand dune

Orbiter: Hybrid treadmill-trampoline that features a flexible running surface made of a suspended, 3/8-inch-thick rubber-cotton-Kevlar belt.

Likes: Offers a spectacularly challenging, fun and minimal-impact cardio workout, as if I were running on a sand dune. The trampoline-like belt, revolving like a tank tread on two tracks and 44 rollers (similar in design to a Woodway), dips at the point where your foot lands, eliminating more than half of the trauma to your joints, says the company, a claim that seems credible. Maximum speed of 8 mph and elevation of 15% is extremely difficult given the extra effort required; your heart rate jumps at lower speeds. According to University of Virginia team doctor Scott Wagner, who owns two, repeated use will increase land speed, correct imbalances and cut injuries. "It engages the hip flexors more and retrains people to use their normal biomechanics," he says. The belt can be adjusted to be firmer or softer. Small 1-horsepower motor uses one-fourth the electricity of a normal treadmill. Elevation and speed buttons are on the handles. Ten custom programs.

Dislikes: None, unless you dislike running at a slower-than-usual pace and in sand.

Price: $9,950. (281) 424-5220; http://www.orbitertreadmill.com

Light on your feet

Pneumex Pneu-Track PT 50: Super-tread with a maximum speed of 30 mph and an overhead frame that "unweights" a runner by using a cable-attached vest harness to provide slight lift.

Likes: It's impact-reducing and exhilarating; you feel like the Road Runner. You lift some of your body weight, up to a maximum of 300 pounds, via a digital-readout dial control. At 70% of my normal body weight, I ran pain-free on a semi-injured right knee for 40 minutes at various speeds, including 10 minutes at 10 mph — way beyond my ability. "Every pound you take off is 4 to 5 pounds off your knee and spine at foot strike, due to G-forces," says Ron Cram, director of Sequoia Physical Therapy in Orange. The vest did not encumber my arm swing or normal breathing. The system works for sideways and backward training as well as high-performance over-speed workouts, which give athletes greater leg turnover. If you already have a treadmill, save with the Pneu-Weight Unweighting System, a stand-alone, overhead structure with cable-vest that can also unweight 300 pounds ($7,350). [For the record: An earlier version of this story misspelled Ron Cram's last name as Kram.]

Dislikes: The vest, cinched very tight under my ribs, became a bit uncomfortable at 15 mph.

Price: $26,500. (800) 447-5792; http://www.pneumex.com

Running on the moon

AlterG Anti-Gravity M320 Treadmill: Device that lightens a runner's weight by as much as 80% by lifting the body in a bubble of air pressure.

Likes: Similar exhilaration of lightweight speed as the Pneumex except that nothing is touching your body from the waist up. To get in, you pull to waist-level a large gasket that resembles the spray-skirt of a kayak, then zip it in place on the airtight enclosure over the treadmill's running surface. You control the amount of unweighting at a panel at the front of the tread. Maximum speeds are 12 mph forward and 3 mph in reverse, with a 15% incline. It can handle a body of up to 400 pounds and 6 feet, 6 inches. (Pro teams use the $75,000 P200 model, set up for 18 mph sprinters and 7-foot basketball players). According to Darwin Fogt of Evolution Physical Therapy in Playa del Rey, the device has "greatly accelerated the rehab of injured Lakers and Kings players, because they can dial in exactly how much body weight they can bear without pain during exercise."

Dislikes: You can't see your feet or the treadmill's side rails. I wanted to step off momentarily but couldn't see where to step.

Price: $28,000. (510) 270-5900; http://www.Alter-G.com

Escaped gym rat

Elliptigo 8S elliptical bicycle: A stationary-handled, 40-pound elliptical machine on wheels that is targeted at runners.

Likes: Superb all-body cardio workout, great all-day fun and a motion fairly close to running. Like the similarly exhilarating Street Strider reviewed here a couple of months ago but with a slightly quicker learning curve, the Elliptigo's elliptical motion gives you a fast, upright, joint- and back-friendly joyride. It falls just short of full cycling velocities (due to additional wind-resistance and low gearing) but has a much better view than a bike and a very satisfying feeling of speed. Most important, you can ride it all day, or all week, as ultra-running star Dean Karnazes proved a few weeks ago on a trip from San Francisco to Santa Monica. I joined him for the final 8-hour, 100-mile push from Santa Barbara. My iffy right knee didn't hurt a bit as I battled headwinds and the Malibu hills, cranking up the leverage by rocking the handlebars back and forth. "It's enough like running that it's gotten me back into exercise," I was told by 53-year-old John Pilkington of San Diego, an ex-runner with a bad back who never liked bikes but who bought one of these last month.

Dislikes: The Shimano 8-speed hub works well but needs a higher top-end gear. I spun out and lost some speed on the flats and downhills without it.

Price: $2,195. http://www.elliptigo.com

Wallack is the author of "Run for Life: The Breakthrough Plan for Fast Times, Fewer Injuries, and Spectacular Lifelong Fitness." roywallack@aol.com